Ant build.xml: look for ANTLR 3 library in lib folder, download from antlr.org if missing. Remove unnecessary build.properties.
Ant build.xml: allow user settings in user.build.properties
Ant build.xml: rename antlr3.gen.dir property to build.antlr3.dir
Ant build.xml: Move mkdir task to antlr3 target, where the grammars are actually compiled
Ant build.xml: use macro to simplify multiple calls to the tool
Ant build.xml: improved up-to-date checks, remove need to pass "-make" to the ANTLR 3 tool